1. Field of the Invention
The present invention relates to a method for reducing a background in a raster image, the raster image comprising pixels having a lightness value, the background indicating a deviation between a lightness value of a subset of pixels in the raster image and a maximum lightness value of raster image input values.
The present invention further relates to a print system, comprising a scanner for converting a hardcopy original into a digital raster image, a programmable electronic circuit for processing the digital raster image into an output raster image, and a print engine for marking a receiving medium in accordance with the output raster image.
2. Description of Background Art
Digital image processing methods have proliferated since their inception a number of decades ago. Depending on the application and the available resources in terms of processing capacity, time and memory, a selection of an appropriate algorithm may be made. These methods have profoundly affected the field of copying, wherein, in its traditional form, light sensitive materials were exposed to light stemming from an illuminated original hardcopy. In its present-day form, an original hardcopy is converted into a raster image, comprising pixels with digital values that indicate a local property of the image, such as color, optical density, or lightness. Digital image enhancement methods are applied to improve some of the image characteristics, whether or not in dependence of an original type, and user preferences. Subsequently, the enhanced digital raster image is converted to print data that are used in a print engine for marking a receiving medium, such as paper or any other substantially flat material that is suitable for receiving an image.
Many hardcopy images comprise dark information on a light background. Such images include text information or line drawings for engineering purposes on white paper. Very old hardcopy originals sometimes have light information on dark background, which is readily inverted once the images are brought into the digital domain. Therefore, in this description, information will be considered to be dark relative to a light background. Depending on the quality and age of the hardcopy, the light background may have a varying lightness. Furthermore, a utilized hardcopy may contain stains or locally darkened areas, obfuscating the information that was supposed to contrast with the background. Especially light sensitive materials, such as used in diazo whiteprint processes, may have degraded in this way. In extreme cases, the background may be locally darker than the information on other parts of the hardcopy.
In the process of converting an original hardcopy into a digital raster image, e.g. by application of a scanner, either of a flatbed type or a transport type, a design choice is made for a range of the digital lightness values. Depending on the lightness values that are obtained for a specific original, the pixel values are scaled to a proper, predetermined output range, as indicated e.g. by W. K. Pratt, Digital Image processing, John Wiley & Sons, 1978, p. 308, which leads to a discrete image contrast enhancement. However, a representative value for a maximum occurring value in the specific original is required for the indicated scaling, that converts this maximum occurring value to a maximum value of the predetermined output range. This maximum occurring value, or background parameter, is often obtained from an analysis of a histogram of occurring digital values for pixels of the image, or related lightness values. In this kind of algorithm, one maximum occurring value, a global maximum occurring value, is used for scaling all pixels of the image. Such a histogram analysis can only be made if all pixel values are available, which is after the capturing of the complete image is finished.
In some capturing processes, not all pixel values of the full original are available. This may be the case in a capturing process of large technical documents, such as architectural and structural drawings, using a transport type scanner. If the processing already starts, before the last part of the original is captured, a firstly scanned part of the original may be regarded as representative for the whole image. As a consequence, depending on the original, a resulting digital raster image may depend on the way the original is introduced in the capturing device. A moving average method, which adapts a dynamic background parameter to pixel values in new scanlines, alleviates this dependency.
Images of originals having a locally varying background, may be processed in portions, or tiles. Around every single pixel, a tile comprising a number of pixels is defined. A histogram of values of pixels within the tile is made in order to derive a local maximum occurring value, or a local background parameter, which is then used to scale the single pixel within the tile. This tile, or window, comprises a sufficient number of pixels to compose a reliable histogram. This kind of moving window method for processing pixels is commonly applied for locally adapting an image characteristic. An example is the contrast enhancement method in U.S. Pat. No. 7,515,748. However, these methods are quite computationally intensive, which means they are not very suitable for productively processing large raster images from technical documents with limited resources.